Device and process for the read-synchronization of video data and of ancillary data and associated products

ABSTRACT

The present invention relates to a device ( 1 ) and a process for the read-synchronization of video data and of ancillary data, these data being recorded in a storage space ( 15 ) in multiplexed and chronological form.  
     The synchronization device comprises modules for inputting ( 2 ) and demultiplexing ( 3 ) the recorded data, the video and ancillary data being respectively transmitted to video reading ( 20 ) and ancillary ( 30 ) assemblies, which read and communicate these data. It also comprises an apprising module ( 4 ) capable of obtaining information from the video reading assembly ( 20 ) as regards instants of communication of video data, and an adaptation module ( 5 ) designed to adjust instants of communication by the ancillary reading assembly ( 30 ) of the ancillary data, as a function of the information obtained by the apprising module ( 4 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention pertains to a device and to a process forthe read-synchronization of video data and of ancillary data, these databeing recorded in a storage space in multiplexed and chronological form,as well as to associated products. It applies in particular to themanagement of the synchronization of teletext subtitles in record readcontrol mode (“play-back”).

[0003] 2. Description of the Related Art

[0004] In a digital television decoder associated with storagefunctionalities, various streams of audio, video, teletext/subtitlestype and of other types such as described in particular in the MPEG(standing for Moving Picture Experts Group) and DVB (standing forDigital Video Broadcasting) standards may be stored in the form of apartial stream on a storage space, for the purpose of being played backand decoded at a later instant. The digital television decoder is forexample equipped with a personal video receiver or PVR (standing forPersonal Video Recorder), furnished with a built-in hard disk on whichthe streams are stored.

[0005] These various streams are customarily transported in an MPEGtransport stream, demultiplexed by a demultiplexer, and transmittedrespectively to elementary decoders for audio, video, teletext orsubtitles decoding, or transmitted to systems for processing data (MPEGsections or other types of data). The streams transmitted to a decodingfacility may be intended to be used synchronously. This is the case forexample for audio and video information which is interrelated bytemporal synchronization information in the form of time labels or PTSs(standing for Program Time Stamps). The utilization of this information,combined with the updating of a local clock by means of time referencesignals denoted PCR (standing for Program Clock Reference), in thepresent case at the decoding level, makes it possible to display imagessynchronously with the sound. This method is well known and employed indigital decoders.

[0006] The transmission of the synchronization information is howevernot obligatory according to the DVB standard in the case ofsubtitles/teletext data. Indeed, the synchronization for such data, therequirement for which is much less precise than in the case of audio, iscarried out de facto by quasi-immediate decoding (a few tens of ms atthe most) of the data received at the output of the demultiplexer. Moreprecisely, by considering two consecutive packets transmitted in thestream and relating respectively to the video and synchronous subtitleson transmission, the corresponding error in terms of synchronizationresults:

[0007] from the duration required to recover the whole of the compressedvideo image;

[0008] from the duration required to establish a nominal video decodingregime, that is to say from the duration of filling of a transportbuffer memory of the video elementary decoder so that this buffer cannotbe starved of data (“underflow”) or be too full of data (“overflow”);

[0009] from the duration required for the decoding of the data indisplayable form and for the displaying of the data in the video frames;

[0010] from the duration required to recover all of the associatedsubtitles/teletext data;

[0011] and from the duration required to decode and compose subtitlesdisplayable by the screen display function or OSD (standing forOn-Screen Display) of the decoder.

[0012] It is these durations aggregated that typically extend over a fewtens of ms. Hence, within the context of the utilization of subtitlestreams in nominal direct decoding mode in accordance with an antennasignal, the presence or otherwise of synchronization information inthese streams is of no great interest, the taking into account of anaverage error sufficing to gain precision.

[0013] In the case of a partial stream played back from a storagefacility, on the other hand, the video data are decoded in tempo withtheir arrival in the video decoding buffer. The principle generally usedduring the reading out of data played back from a storage facilityconsists in buffering (that is to say placing in buffer memory) thereading times for the storage facilities by maximally filling all thedata buffers used, and in particular the video decoding buffer. In thisway, any reading fault as regards a storage facility entailing a delayin the reading of these data is not visible. Thus, all the data—audio,video, subtitle etc—read out from a storage facility is provided to thedemultiplexer, with feedback control directed by the level of fill ofthe video decoding buffer.

[0014] In the case where no pre-dispatch synchronization mechanism (suchas PTS) is employed for the subtitles, the latter are decoded as soon asthey are available at the output of the demultiplexer, in the same wayas in direct reception. Video data that are not yet decoded mighttherefore still be present in the video decoding buffer, whereas thecorresponding subtitles are already being displayed. In practice, thesize of the video decoding buffer and the customary video compressionbit rates (more than 300 kbytes for the size of the decoding buffer andan MPEG video bit rate of the order of 2.0 Mbits/s) are such that it ispossible to have several seconds of gap between these subtitles and theas yet undecoded image. The coherence of display of the subtitles withrespect to the video is then seriously compromised.

[0015] A possible solution to this problem would consist in introducingtime labels on reception, before recording the data in the storagespace. These labels would then serve to read the subtitlessynchronously. Thus, a conceivable method consists in taking account ofthe video PTSs to determine the time labels of the subtitles, byanalyzing the stream so as to edit the video transport packets and thepackets of elementary video streams or PES (standing for PacketElementary Streams) and by feedback controlling the local clock to thevideo PCR signals. These operations are relatively complex and unwieldyto implement, and increase the storage room required in order to be ableto store the time labels created for the subtitles. They moreoverrequire a knowledge of the video packet identifiers (called PID standingfor Packet Identifiers) and lead to difficulties of implementation whenthe video PESs are scrambled.

[0016] Another method with introduction of time labels before storagewould consist in systematically affixing such additional labels to allthe packets on the basis of a local clock, while transferring theanalysis operations to the reading. Thus, during the reading of the datarecorded in the storage space, the recorded data would be recovered,they would be transmitted synchronously by virtue of these additionallabels from a first module to a second module, and then the datareceived by the second module would be processed like those receiveddirectly from an external source. However, this technique requiresadditional storages for storing all the time labels, and raises thecomplexity of recording and of reading by requiring the implementationof specific procedures that are relatively unwieldy in terms of meansand operations.

[0017] It is also known, when recording audiovisual data and ancillaryinformation on optical discs, such as for example DVDs (DigitalVersatile Discs) or CDs, (Compact Discs) to introduce time labelsassociated with audiovisual and ancillary data.

[0018] Thus, patent U.S. Ser. No. 2001/0005447, discloses methods formultiplexing and demultiplexing audiovisual digital data and characters.Each recorded packet of these data includes a time label indicative of adecoding time or display time. During reproduction of the recorded data,a time label supervisor controls the synchronism of decoding of severaldecoders respectively associated with the video, the audio and withsubtitles, in relation to the time labels of the data packets.

[0019] A technique of this type can be utilized for example in theembodiment set forth in patent U.S. Pat. No. 6,122,434. This documentrelates to a medium and to an apparatus for recording information,involving various levels of permission of reproduction. It is centredaround parental supervision applied to optical discs, for which thelevels of permission differ according to country. The disclosurepertains in particular to systems for recording and reproducing DVDs,that make it possible respectively to record, and to reproducesynchronously, video, audio, subimage and information supervision data.The synchronization of the data extracted from the recording medium isperformed by buffer memories before decoding, respectively associatedwith the above data categories. The modus operandi in respect of thesemultiple synchronizations not being made explicit, synchronization bytime labels seems to be the appropriate solution.

[0020] However, the introduction of time labels for all the datanecessitates complex means, in particular hardly appropriate forrecordings of streams received in terminals, and requires extrastorages.

[0021] An additional difficulty is posed by the use of special effectsmodes (or trick modes) on the basis of the recordings of the data in thestorage space, even in the presence of time labels received with thesubtitling data. Specifically, as soon as the speed of transmission ofthe video data is decelerated or accelerated with respect to the normalspeed, it becomes necessary to make corrections to the time labelsassociated with the subtitles, as a function of this speed oftransmission of the video data. Such a correction of synchronizationnecessitates specific processing adapted to trick modes, which may inparticular rely on adjusting a local counter's value elapsed since thearrival of the last video PTS. The complexity of the mechanisms able toutilize the recorded data is therefore heightened.

[0022] What is more, in the absence of time labels associated with thesubtitling data, the problems mentioned above in respect of a normalreading speed are further heightened in slow-motion mode, since thedelay between the displaying of the subtitles and that of thecorresponding video images becomes still greater.

SUMMARY OF THE INVENTION

[0023] The present invention relates to a device for theread-synchronization of video data and of subtitles on the basis of astorage space, enabling synchronized reading at the cost of a tinyexpense in terms of storage and operations. Moreover, this device may berelatively simple to implement. It is in particular capable of beingapplied to video streams and subtitle streams received and recorded,without these subtitles being furnished with time labels. Thus, thedevice of the invention may be capable of remedying the absence ofsynchronization information in teletext DVB type streams, in cases wherepartial transport streams are played back from a hard disk moreprecisely for the synchronization of subtitles transported in teletexttype streams.

[0024] The synchronization device of the invention may also have asadvantage, in preferred embodiments, that it allows in particularadaptation of synchronization to special effects modes, whether thespeed of reading from the storage space is accelerated or decelerated.

[0025] More generally, the invention concerns a device for theread-synchronization of video data and of ancillary data, these databeing recorded in a storage space in multiplexed and chronological form,having the qualities mentioned above.

[0026] The invention also relates to a synchronization processcorresponding to the synchronization device of the invention, a videodecoding system comprising such a device and an associated computerprogram.

[0027] The invention finds applications not only in subtitling, but alsoin respect of synchronization pertaining especially to the production ofdata (sounds, pictures, commands etc.) in connection with interactiveprograms, and to the displaying or sound transmission of informationtacked onto video programs, provided that synchronization is performedlive.

[0028] To this end, a subject of the invention is a device for theread-synchronization of video data and of ancillary data associated withthese video data, with a view to a synchronized communication of thosedata, the video and ancillary data being recorded in a storage space inmultiplexed and chronological form. The synchronization devicecomprises:

[0029] a module for inputting the recorded data in the storage space,

[0030] and a module for demultiplexing the video and ancillary data,which is designed to transmit the video data to a video reading assemblyand the ancillary data to an ancillary reading assembly, these readingassemblies being intended to read and to communicate the data.

[0031] According to the invention, the synchronization device alsocomprises:

[0032] a module for apprising as regards instants of communication ofthe video data, which is capable of obtaining information originatingfrom the video reading assembly as regards these instants ofcommunication,

[0033] and a module for adaptation of communication of the ancillarydata, which is designed to adjust instants of communication by theancillary reading assembly of the ancillary data, as a function of theinformation obtained by the apprising module.

[0034] The term “communication” of the data is understood to mean anyoperation of transmission to a module external to the reading assembly,which may therefore lead in particular to screen display or to soundproduction at the local level (presentation of data to a televisionviewer, for example), or to dispatching into an external circuit of thedata read (transmission from a centralized system to a suite oftelevision sets of a property or of an enterprise, for example).

[0035] Moreover, the information obtained by the module for apprising asregards the instants of communication of the video data may be indirectand partial. It may in particular be information as regards durations oftransit through the video reading assembly.

[0036] The adjusting of instants of communication of the ancillary datamay likewise be indirect. It is thus possible to act in particular byintroducing adaptation delays either as regards the instants oftransmission to the ancillary reading assembly, or as regards thecommunication at the output of this assembly, or else possibly asregards intermediate steps, for example the decoding of current elementsof the ancillary data once they have reached the ancillary readingassembly. Combined solutions are also possible.

[0037] The device of the invention is particularly unexpected in thelight of existing methods, since it does not implement any extra timelabel for the ancillary data. By contrast, it best utilizes the actualoperation of the mechanisms for reading the video and ancillary data, bybeing able to take account of the delays caused by the video-bufferaccumulation of the video data before decoding. This utilization isperformed in real time when the data recorded in the storage space areread. More precisely, the synchronization relies on a form of feedbackcontrol of the instants of communication of the ancillary data, as afunction of the instants of communication of the video data.

[0038] This device may be especially advantageous in the light ofexisting techniques, or in relation to what would appear to be naturallytempting for a person skilled in the art (introduction of time labels):

[0039] synchronization possible even in the absence of any time labelfor the ancillary data received and recorded;

[0040] no specific operation necessary before recording of the data inthe storage space;

[0041] no need to record excess information (savings in storage);

[0042] utilization possible of any recording of video and ancillary datain multiplexed and chronological form, or even without any specificprovision regarding recording (flexibility of use);

[0043] and possibility of reading video according to a customarytechnique, without the need for additional operations of syntacticstream analysis.

[0044] Preferably, the apprising module is capable of dynamicallyobtaining the information as regards the instants of communication ofthe video date, in such a way that the adaptation module is able to takeaccount of the accelerations and decelerations of reading of these data.This dynamic obtaining may in particular be carried out by means of asufficiently temporally fast sampling of measurements. For example,measurements are performed every video frame.

[0045] In another embodiment, the apprising module is designed to obtainthis information statically, then to use it subsequently in a systematicmanner. It is in particular possible to impose a fixed time offset onthe instants of communication of the ancillary data, on the basis ofinitial measurements. In an improved form of this embodiment, themeasurements are redone automatically in the case of accelerated ordecelerated running speed (these operations being triggered by a changeof speed or of special effects mode signal). The benefit of this staticembodiment, which is less precise and flexible of adaptation than thedynamic one, is to further reduce the quantity of operations required.

[0046] In advantageous forms of the data, the modules are designed sothat the ancillary data comprise:

[0047] subtitles;

[0048] data relating to at least one interactive program (pictures,sounds, messages, commands etc);

[0049] or a combination of the above two types of data.

[0050] The modules are advantageously designed so that the data arerecorded in digital form, preferably according to the DVB standard.

[0051] According to a preferred form of apprising, the video readingassembly comprising:

[0052] at least one buffer memory intended to provisionally containcurrent elements of the video data,

[0053] a video decoding unit intended to successively decode the currentelements on output from this buffer memory;

[0054] and a communication unit intended to communicate with the decodedcurrent elements, the apprising module is designed to obtain dynamicallyat least information relating to the content of this buffer memory.

[0055] This form of apprising may turn out to be particularly effectiveand relevant. These advantages are due to the fact that the content ofthe video buffer memory is representative of the time offset as regardsthe communication of the video data with respect to the ancillary data.Now, it is precisely this offset that is problematic for thesynchronization between the video and ancillary data.

[0056] The apprising module is then preferably such that thisinformation is chosen from among measurements giving:

[0057] a quantity of the current elements present in this buffer memoryfor the video;

[0058] an incoming stream of the current elements entering this buffermemory;

[0059] an outgoing stream of the current elements leaving this buffermemory;

[0060] a number of the current elements leaving this buffer memory sincea predefined instant of transmission of the ancillary data;

[0061] and any combination of at least two of the above measurements.

[0062] In a first advantageous form of such an apprising module, thelatter is designed to measure a number B of bytes present in the videobuffer, as well as a video bit rate D at the output of the buffer, asquotient of the level of the buffer over the stream of video data thatare dispatched thereto. The apprising module is intended to deduce fromthese values B and D a video offset delay, given by the ratio B/D(approximate delay between the receipt of the video data in the videoreading assembly and their decoding—with the exclusion of the last datapackets received by the decoding unit). This video decoding delay,possibly supplemented by means of other parameters, is then utilizableby the adaptation module.

[0063] In a second advantageous form of embodiment, the apprising moduleis designed to establish the number B of bytes present in the videobuffer, and to determine the outgoing video stream leaving the buffer(consumption of data). The apprising module is then capable of comparingthe number of bytes consumed in the buffer with N and of transmitting atrigger signal to the adaptation module for communicating the ancillarydata, when the consumption of B bytes in the video buffer since thestart of the count of the bytes is completed. This embodiment makes itpossible to ensure better precision than the first in the fast forwardor slow motion modes, since the value of the bit rate is not considered.

[0064] In a third advantageous form of embodiment, the apprising moduleis designed to measure the number B of bytes present in the video bufferand the speed of decoding of the video decoding unit, and to refer themto one another. The delay obtained gives a video offset approximatelyrepresenting the time span elapsed between the receipt of the video datain the video reading assembly and their complete decoding. Thus, theactual bit rate is immediately modified as soon as the decoding speedchanges, thereby avoiding the need to wait for this value of bit rate tostabilize over time.

[0065] In a more sophisticated version of any one of the previousembodiments pertaining to the apprising of particulars at least relatingto the content of the video buffer, the apprising module is designed toapply a low-pass filter to values estimated dynamically by sampling,such as values of video bit rate (first embodiment) or of decoding speed(third embodiment).

[0066] In a fourth advantageous form of embodiment, advantageouslycombined with any one of the previous ones, a fixed value of offsetincorporating various accessory delays is added to a delay estimationbased on the content of the buffer. This value may in particular takeaccount:

[0067] on the one hand positively, of the durations of recovery of acompressed video image, of decoding of the data in displayable form, ofdisplaying of the data in the video frames,

[0068] and on the other hand negatively, of the durations of recovery ofa set of ancillary data such as subtitles associated with a video image,of decoding and of composition of these subtitles displayable by the OSDfunction of the decoder.

[0069] In other forms of apprising as regards the instants ofcommunication of the video data by the video reading assembly, theapprising module is designed to bring about the triggering of returnsignals upon instants of decoding or of communication of video data.These return signals may then be utilized directly in respect oftemporal adjustments of the instants of communication of the ancillarydata. These embodiments, able to afford greater precision than theprevious ones, nevertheless require more complex mechanisms (recognitionof the images to be signalled).

[0070] The invention relates moreover to a video decoding system,comprising a synchronization device in accordance with any one of theembodiments of the invention.

[0071] This video decoding system advantageously comprises a storagespace, this storage space consisting preferably of a hard disk.

[0072] The subject of the invention is also a process for theread-synchronization of video data and of ancillary data associated withthese video data, with a view to a synchronized communication of thesedata, the video and ancillary data being recorded in a storage space inmultiplexed and chronological form. In this process, the recorded videodata and ancillary data are demultiplexed by transmitting the video datato a video reading assembly and the ancillary data to an ancillaryreading assembly, the reading assemblies being intended to read and tocommunicate these data.

[0073] According to the invention, instants of communication by theancillary reading assembly of the ancillary data are adjusted as afunction of the information obtained originating from the video readingassembly as regards instants of communication of the video data.

[0074] The synchronization process is preferably intended to beimplemented by means of a synchronization device in accordance with anyone of the embodiments of the invention.

[0075] The invention also applies to a computer program productcomprising program code instructions for the execution of the steps ofthe process according to the invention, when this program is executed ona computer. The expression “computer program product” is understood tomean a computer program support, which may consist not only of a storagespace containing the program, such as a disc or a cassette, but also ofa signal, such as an electrical or optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The invention will be better understood and illustrated by meansof the following wholly non-limiting exemplary embodiments andimplementations, with reference to the appended figures in which:

[0077]FIG. 1 is a diagram of the principle of a video decoding systemcomprising a synchronization device in accordance with the invention;

[0078]FIG. 2 shows a multiplexed string of video data representing Nimages and of corresponding subtitling data, recorded in a hard disk ofthe video decoding system of FIG. 1;

[0079]FIG. 3 represents an initial step of read-utilization by thedecoding system of FIG. 1, of the data string of FIG. 2;

[0080]FIG. 4 represents a later step of re-utilization by the decodingsystem of FIG. 1, of the data string of FIG. 2;

[0081]FIG. 5 represents a subsequent step of read-utilization by thedecoding system of FIG. 1, of the data string of FIG. 2;

[0082] and FIG. 6 represents a last step of read-utilization by thedecoding system of FIG. 1, of the data string of FIG. 2.

[0083] In FIG. 1, the modules represented are functional units which mayor may not correspond to physically distinguishable units. For example,these modules or some of them may be grouped together into a singlecomponent, or constitute functionalities of one and the same piece ofsoftware. A contrario, certain modules may possibly be composed ofseparate physical entities.

DESCRIPTION OF THE PREFFERED EMBODIMENTS

[0084] A video decoding system 10 (FIG. 1), for example of thetelevision type equipped with a PVR, is designed to receive audiovisualprograms received in the form of streams F, and to transmit themdirectly to a television viewer or record them in a storage space 15 soas to render them accessible later, the two operations possibly beingperformed simultaneously. In FIG. 1, only the functionalities connectedwith recording are represented.

[0085] The video decoding system 10 comprises, in addition to thestorage space 15 consisting for example of a hard disk:

[0086] a module 11 for receiving the audiovisual streams F;

[0087] a module 12 for recording the data of these streams F on the harddisk 15;

[0088] a module 13 for extracting the data recorded on the hard disk 15;

[0089] a synchronization device 1, able to demultiplex the extracteddata and to distribute them to appropriate decoders according to theirtypes, and to monitor a synchronized communication of these varioustypes of data to the television viewer;

[0090] a video reading assembly 20, designed to decode the data of videotype;

[0091] an ancillary reading assembly 30, designed to decode the data ofsubtitles and teletext type;

[0092] and a display module 14, intended to communicate the decoded datato the television viewer.

[0093] The video reading assembly 20 comprises a buffer 21 foraccumulating the video data before decoding, an elementary video decoder22, and a unit 23 for communicating the decoded data to the displaymodule 14. In a similar fashion, the ancillary reading assembly 30comprises a buffer 31 for accumulating the subtitles and teletext databefore decoding, an elementary ancillary decoder 32, and a unit 33 forcommunicating the decoded data to the display module 14.

[0094] The synchronization device 1 for its part is furnished with amodule 2 for inputting the data extracted from the hard disk 15 and ademultiplexer 3, designed to identify and disperse these data to thereading assemblies 20 and 30. It also comprises an apprising module 4,capable of obtaining information originating from the video readingassembly 20 as regards instants of display of the video data to thetelevision viewer, and an adaptation module 5, designed to adjustinstants of display of the subtitles data as a function of thisinformation.

[0095] In the embodiment described, the adjustment is done byintroducing delays of display as regards the already decoded subtitlingdata, by acting on the ancillary reading assembly 30 (solid line). In avariant embodiment, the adaptation module 5 is designed to act upstreamof the decoding on the demultiplexer 3, by introducing delays oftransmission of the subtitling data to the ancillary reading assembly 30after demultiplexing (dashed line). Such an embodiment may for examplebe obtained by using an elementary buffer associated with thedemultiplexer 3, upstream of the reading assembly 30.

[0096] Several schemes of embodiment of the synchronization device 1will now be set forth, with reference to FIGS. 2 to 6, through anexample implementation.

[0097] In this example, a data string C recorded on the hard disk 15(FIG. 2) is composed of video data in the form of video images V1, V2 .. . VN and of subtitling data in the form of subtitles ST1 and ST2,these video and subtitling data being multiplexed. More precisely,subtitle ST1, interposed between images V1 and V2, and subtitle ST2,disposed downstream of image VN, are designed to begin to be transmittedrespectively with images V1 and VN. Thus, the N video images V1 to VNhave to be displayed in succession between the dispatching of the twosubtitles ST1 and ST2, thereby determining a duration of gap betweentheir display.

[0098] For the sake of simplification in the presentation, it is assumedthat the size of the set of video images V1 to VN corresponds to thecapacity of the video decoding buffer 21, and that these images areintra-type MPEG images, sufficient in themselves for decoding. Thispurely illustrative example may readily lead to the understanding ofmore complex cases. Moreover, the explanations are centered hereinbelowon the synchronized displaying of the second subtitle ST2, but it mayreadily be verified that they are also applicable to the synchronizeddisplaying of subtitle ST1, which is the first subtitle transmittedafter a startup.

[0099] During reading from the hard disk 15 (FIG. 3), image V1 isprovided to the buffer 21 by the demultiplexer 3 and is decoded by thedecoder 22. The corresponding subtitle ST1 is for its part provided tothe buffer 31 and decoded by the subtitles decoder 32.

[0100] The procedure for decoding data originating from the hard disk 15being such that the video decoding buffer 21 must be maximally filled,the data therefore continue to be extracted from the hard disk 15 so asto be transmitted to the demultiplexer 3 and to the two decoding buffers21 and 31. The bit rate of transfer of the video data from the hard disk15 to the decoding video buffer 21 is, in accordance with the customarysituation, much greater than that of the video decoding. Typically, thetransfer and video decoding bit rates are respectively of the order of40 Mbits/s and 2 Mbits/s. It may therefore be considered that in mostrealistic cases one is in the situation where the buffer 21 is filled upto video image V(N−1), whereas the decoded image is only image V1 or V2.

[0101] When video VN then arrives in the video decoding buffer 21 (FIG.4), subtitle ST2 reaches the subtitles decoding buffer 31 and isimmediately decoded. One is then in the case where subtitle ST2 is readyto be displayed considerably in advance, potentially of almost theentire content of the video decoding buffer 21 (to within the errorsindicated above). Typically, this advance is of the order of 300kbytes/[2 Mbits/s] (video quantity to be decoded over decoding speed),that is to say of more than one second. What is more, this lag decreasesif the video bit rate increases, and increases if the video bit ratedecreases.

[0102] With management of the PTSs for the subtitles, this offset wouldnot be visible since the subtitles decoder 32 would decode subtitle ST2and would wait for the presentation instant given by the correspondingtime label to have arrived, in order to actually display it on thescreen. Subtitle ST2 would then be displayed with video VN, as intended.

[0103] The synchronization device 1 does without these PTSs, whether ornot they are present in the data extracted from the hard disk 15. To dothis it applies the algorithm described hereinafter. Let T1 be the timeof arrival of subtitle ST2 in the decoding buffer 31, B the number ofbytes present in the video decoding buffer 21 at this instant T1 and Dthe actual video bit rate, obtained by regularly sampling the level ofthe video buffer 21 and the number of data dispatched to this buffer 21.The instant of display T2 of subtitle ST2 is calculated by estimatingthe instant of video decoding corresponding to the consumption of the Bbytes by the video decoder 21:

T 2 =T 1 +B/D

[0104] The instant T2 thus calculated gives the moment at which decodedsubtitle ST2 is to be displayed. Thus, the already decoded subtitle ST2is placed on standby while waiting for images V2 to VN to be decoded inturn (FIG. 5), and is displayed at the appropriate moment T2,approximately at the same time as image VN (FIG. 6). To do this, forexample, the communication unit 33 keeps subtitle ST2 until a signaltriggered by the adaptation module 5, which brings about transmission tothe display module 14.

[0105] In an effective scheme of embodiment, the sampling of the levelof the video buffer 21 is done every video frame, i.e. every 20 ms for astandardized European decoding system of PAL/SECAM type (50 Hz, standingfor Phase Alternate Line/SEquentiel Couleur A Mémoire), and every 16.6ms for the standardized systems of NTSC type (60 Hz, standing forNational Television System Committee).

[0106] This method operates with good precision not only at constantspeed, but also in the fast forward or slow motion modes, since the bitrate D adapts to the reading speed.

[0107] As indicated above, it is possible to verify that thesynchronization of the first subtitle ST1 after startup with the videoimage V1 is also ensured by the method described. Specifically, thecapacity B of the video buffer 21 upon the arrival of subtitle ST1corresponds to image V1. Hence, the estimated lag in displaying subtitleST1, given by the ratio B/D, corresponds approximately to the lag indisplaying the image V1 from the receipt of subtitle ST1 in theancillary decoding buffer 31.

[0108] In a first variant, one regularly samples the consumption of datain the video decoding buffer 21. In this way, the instant T2 is detectedby comparing the number of bytes consumed in the video buffer 21 fromthe instant T1 with the number of bytes B present in this buffer 21 atthis instant T1. In an advantageous embodiment, this sampling is doneevery video frame. The first variant makes it possible to ensure betterprecision in the fast forward or slow motion modes, since the value D ofthe bit rate is no longer to be considered.

[0109] In a second variant, a low-pass filter is applied to theestimated bit rate value.

[0110] In a third variant, the bit rate value is corrected by takingaccount of the decoding speed information. Hence, the actual bit rate isimmediately modified as soon as the decoding speed changes, therebyavoiding the need to wait for this bit rate value to stabilize overtime.

[0111] In a fourth variant, a fixed value of offset as regards theinstant of display T2 is applied, in such a way as to incorporate anestimation of time errors not taken into account by the initialapproximation as regards this instant T2.

[0112] According to other exemplary implementations, the lag in theprocessing of subtitle ST2 is introduced:

[0113] after demultiplexing, before transmission to the subtitles buffer31;

[0114] after placement in buffer 31, before decoding by the decoder 32;

[0115] or in the display module 14, before receipt of a display triggersignal originating from the adaptation module 5.

1. Device for the read-synchronization of video data and of ancillarydata associated with said video data, with a view to a synchronizedcommunication of said data, said video and ancillary data being recordedin a storage space in multiplexed and chronological form, saidsynchronization device comprising: a module for inputting said datarecorded in the storage space, and a module for demultiplexing saidvideo data and ancillary data, which is designed to transmit said videodata to a video reading assembly and said ancillary data to an ancillaryreading assembly, said reading assemblies being intended to read and tocommunicate said data, wherein said device also comprises: a module forapprising as regards instants of communication of the video data, whichis capable of obtaining information originating from the video readingassembly as regards said instants of communication, and a module foradaptation of communication of the ancillary data, which is designed toadjust instants of communication by the ancillary reading assembly ofthe ancillary data, as a function of the information obtained by theapprising module.
 2. Synchronization device according to claim 1,wherein that the apprising module is capable of obtaining saidinformation dynamically, in such a way that the adaptation module isable to take account of the accelerations and decelerations of readingof the video data.
 3. Synchronization device according to claim 1,wherein said modules are designed so that said ancillary data compriseat least one subtitle.
 4. Synchronization device according to claim 1,wherein said modules are designed so that said ancillary data comprisedata relating to at least one interactive program.
 5. Synchronizationdevice according to claim 1, wherein said modules are designed so thatsaid data are recorded in digital form, preferably according to the DVBstandard.
 6. Synchronization device according to claim 1, wherein thevideo reading assembly comprising at least one buffer memory intended toprovisionally contain current elements of the video data, a videodecoding unit intended to successively decode said current elements onoutput from said buffer memory and a communication unit intended tocommunicate said decoded current elements, the apprising module isdesigned to obtain dynamically at least information relating to thecontent of said buffer memory.
 7. Synchronization device according toclaim 6, wherein the apprising module is such that said information ischosen from among measurements giving: a quantity of the currentelements present in said buffer memory; an incoming stream of thecurrent elements entering said buffer memory; an outgoing stream of thecurrent elements leaving said buffer memory; a number of the currentelements leaving said buffer memory since a predefined instant oftransmission of the ancillary data; and any combination of at least twoof said measurements.
 8. Video decoding system comprising asynchronization device in accordance with claim
 1. 9. Video decodingsystem according to claim 8, wherein it comprises a storage space, saidstorage space consisting preferably of a hard disk.
 10. Process for theread-synchronization of video data and of ancillary data associated withsaid video data, with a view to a synchronized communication of saiddata, said video and ancillary data being recorded in a storage space inmultiplexed and chronological form, in which said recorded video dataand ancillary data are demultiplexed by transmitting said video data toa video reading assembly and said ancillary data to an ancillary readingassembly, said reading assemblies being intended to read and tocommunicate said data, wherein instants of communication by theancillary reading assembly of the ancillary data are adjusted as afunction of the information obtained originating from the video readingassembly as regards instants of communication of the video data, saidsynchronization process being preferably intended to be implemented bymeans of a synchronization device in accordance with claim
 1. 11.Computer program product comprising program code instructions for theexecution of the steps of the process according to claim 10, when saidprogram is executed on a computer.